1. Field of the Invention
The present invention relates to the demodulation of an orthogonal frequency division multiplexing (OFDM) signal, more particularly to a likelihood corrector and a likelihood correction method.
2. Description of the Related Art
Terrestrial integrated services digital broadcasting (ISDB-T) is one use of the OFDM digital modulation system, which effectively combats multi-path fading. The ISDB-T OFDM modulated signal (referred to as an OFDM signal below) has a scattered pilot, that is, pilot symbols are scattered through the signal in the frequency and time directions to provide reference amplitude and phase information for demodulation.
Correct demodulation of an OFDM signal requires correct channel estimation and estimation of the frequency offset between the transmitter and receiver. Maximum likelihood estimation can be used for these purposes. Japanese Patent Application Publication No. 8-293850 describes a method that compares likelihoods generated by two likelihood estimators, and updates the receiver's frequency according to the result, but fails to give details of the likelihood estimation process, saying only that the likelihood estimators employ a method used in decoding a convolutional code. This implies a continuous process rather than a process using scattered pilot signals, which are not mentioned in the disclosure.
When the channel and frequency offset estimates are derived from a scattered pilot signal, it is necessary to allow for the effect of the scattering of pilot signal information, but there is no teaching or suggestion in the above disclosure of how this might be done.